Which one of the following statements about the Coriolis force is not correct?
- (a)It is maximum at the Poles.
- (b)It is absent at the Equator.
- (c)It deflects the wind to the right direction in the southern hemisphere.
- (d)It deflects the wind to the right direction in the northern hemisphere.
Correct — C, It deflects the wind to the right direction in the southern hemisphere. The deflection reverses across the equator, and the paper has simply repeated the northern-hemisphere clause with the wrong hemisphere attached. The school text is explicit: the force 'deflects the wind to the right direction in the northern hemisphere and to the left in the southern hemisphere'. Three of the four options are that same passage broken into pieces — the other two read 'The Coriolis force is directly proportional to the angle of latitude. It is maximum at the poles and is absent at the equator', which is options (a) and (b) word for word, and the northern-hemisphere half of the deflection sentence, which is option (d). Only option (c) has been altered, and the alteration is the tested point.
- (a)It is maximum at the Poles. — True. The Coriolis parameter varies as the sine of the latitude, so it reaches its greatest value at 90 degrees.
- (b)It is absent at the Equator. — True, and consequential. Since the sine of zero degrees is zero, the force vanishes at the equator, the wind blows straight across the isobars into a low, the low fills instead of intensifying — and that is why tropical cyclones do not form on the equator itself.
- (d)It deflects the wind to the right direction in the northern hemisphere. — True, and it is the half of the deflection rule the paper has left intact. Right in the north, left in the south — which is why northern-hemisphere cyclones turn anticlockwise and southern-hemisphere cyclones turn clockwise.
The Coriolis force is an apparent force that arises because the Earth rotates beneath the moving air. It acts perpendicular to the direction of motion, so it changes a wind's direction and never its speed. Its magnitude grows with the speed of the moving air and with the sine of the latitude, which is why it is zero at the equator and greatest at the poles. Together with the pressure gradient force and friction it decides the direction of surface winds; above about two to three kilometres, where friction dies away, the pressure gradient and the Coriolis force alone balance to give the geostrophic wind blowing parallel to the isobars.
This kind of item is best handled by checking whether an option is symmetric or asymmetric. Statements about magnitude — maximum at the poles, zero at the equator — are the same in both hemispheres, so they cannot be hemisphere-dependent and are safe. Statements about direction are hemisphere-dependent by nature, so a paper that gives you two direction statements pointing the same way has told you one of them is wrong. The physical reason for the reversal is the geometry of rotation: seen from above the North Pole the Earth turns anticlockwise, and seen from above the South Pole it turns clockwise, so a parcel of air moving over the surface appears to swing the opposite way in the two hemispheres. The school text credits 'the French physicist who described it in 1844'; Gaspard-Gustave de Coriolis in fact published the analysis in 1835.
- The Coriolis force deflects winds to the right in the northern hemisphere and to the left in the southern hemisphere.
- Its magnitude is directly proportional to the sine of the latitude, so it is maximum at the poles and absent at the equator.
- The deflection is greater when the wind velocity is higher, and the force acts perpendicular to the pressure gradient force.
- At the equator the force is zero, the wind blows perpendicular to the isobars and a low pressure area fills instead of intensifying — which is why tropical cyclones do not form there.
- Above two to three kilometres, free of surface friction, the balance of the pressure gradient and the Coriolis force gives the geostrophic wind blowing parallel to the isobars.
- Assuming the Coriolis force changes wind speed; it acts at right angles to motion and alters direction only.
- Believing the force is stronger in the southern hemisphere because the westerlies there are stronger; that is the absence of land and friction, not a bigger Coriolis parameter.
- Reading 'absent at the equator' as 'weak in the tropics generally'; it is exactly zero only at zero degrees latitude.
Asked as a which-is-not-correct item where one option flips the hemisphere, and as a two-statement item pairing the latitude dependence with the direction rule.
With reference to "Coriolis force", which of the following statements is/are correct? 1. It increases with increase in wind velocity. 2. It is maximum at the poles and is absent at the equator. Select the answer using the code given below:
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer(c) Both 1 and 2
The magnitude half of the same rule, three years later. UPSC affirms the very statements that appear here as options (a) and (b), which is a useful confirmation that the tested point in the CDS item can only be the direction clause.
Assertion (A): Wind patterns are clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere. Reason (R): The directions of wind patterns in the northern and the southern hemisphere are governed by the Coriolis Effect.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is not the correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer(a) Both A and R are individually true and R is the correct explanation of A
The direction half, tested as cause and effect. Wind circulation runs opposite ways in the two hemispheres precisely because the Coriolis deflection reverses — which is what makes the CDS option about a rightward deflection in the south the odd one out.
- practice — not a real PYQ
Tropical cyclones do not form within a few degrees of the equator mainly because
- (a)sea surface temperatures there are too low
- (b)the Coriolis force is negligible there
- (c)the upper atmosphere is too dry there
- (d)vertical wind shear is too weak there
Answer(b) the Coriolis force is negligible there — without it the converging air cannot acquire the spin needed to organise a storm, and the low pressure area simply fills.
- practice — not a real PYQ
The wind that blows parallel to the isobars in the upper atmosphere, where the pressure gradient force is balanced by the Coriolis force, is called
- (a)Katabatic wind
- (b)Geostrophic wind
- (c)Anabatic wind
- (d)Monsoon wind
Answer(b) Geostrophic wind — found above about two to three kilometres, where surface friction no longer acts on the flow.